1. Field of the Invention
This invention relates to a video camera having a negative/positive inverting function for inverting a positive image into a negative image and vice versa.
2. Description of the Related Art
FIG. 1 shows the arrangement of the conventional video camera having the negative/positive inverting function. Referring to FIG. 1, light from an object of shooting 1 comes through a lens 2 and an iris 3 to an image pickup part 4 which includes a solid-state image sensor such as a CCD. The image pickup part 4 then performs a photoelectric conversion process on the light of the object to obtain R (red), G (green) and B (blue) color signals. The R, G and B signals are applied to a processing circuit 5 to be subjected to a white balance adjustment process, a gamma correction process, a matrix process, etc. A luminance signal Y and color-difference signals R-Y and B-Y are obtained through these processes.
These signals Y, R-Y and B-Y are inverted by inversion circuits 6, 7 and 8 and are then applied to an encoder 9. The encoder 9 modulates the signals R-Y and B-Y in a balanced manner. The signals R-Y and B-Y are added together and are further combined with the signal Y into a video signal, which is outputted from the encoder 9.
If the object 1 is a negative photographic film, for example, the processes mentioned above invert the negative image on the film to give a video signal for a positive image. In a case where the object 1 is a normal positive image, the positive image is inverted to give a video signal for a negative image, so that a special effect can be attained.
The details of the inverting process to be carried out by the inversion circuits 6, 7 and 8 can be mathematically expressed as follows:
inverted Y=A-Y (A: a constant) PA1 inverted (R-Y)=-(R-Y) PA1 inverted (B-Y)=-(B-Y)
The luminance signal Y is inverted by subtracting the luminance signal Y from a certain reference level. The color-difference signals are inverted by inverting the polarity of them.
In a case where a negative photographic film is used as the object 1, the positive image obtained from the video signal outputted from the conventional video camera having the negative/positive inverting function has a low luminance level and a small amount of color saturation. The image thus obtained is too dark as a whole and too faint in color. The reason for this is as follows.
As is well known, the negative photographic film has such a characteristic that a relation between the quantity of light incident on the film surface and the blackening amount of the film is nonlinear. This is called a gamma characteristic. The gamma characteristic shows a smaller amount of change in a part having a larger amount of incident light. In other words, the image is printed on the film surface in such a way as to have the image compressed at a part where the luminance level is high. Therefore, in the output obtained by photoelectrically converting the light of the negative film at the image pickup part 4, a part corresponding to a high luminance part of a photographed object is also compressed.
This state is shown in FIGS. 2(a) to 2(e), which are waveform charts obtained by using a gray scale chart as an object of shooting. FIG. 2(a) shows a luminance distribution of a gray scale chart obtained before taking a photograph. Horizontal positions are indicated on the axis of abscissa. Luminance levels are indicated on the axis of ordinate. This gray scale chart consists of a part where the luminance gradually decreases from the left to the right, another part where the luminance gradually increases from the left to the right and a middle part having a high luminance area vertically extending in a strip-like shape with black parts on both sides of the high luminance part. The luminance distributions of these parts are shown as obtained on one and the same plane.
FIG. 2(b) shows an output signal of the image pickup part 4 which is, for example, the G signal. The output signal corresponds to a distribution of blackening amounts of a negative film which is used as the object 1. The distribution is shown upside down, with respect to FIG. 2(a). Further, as mentioned above, the lower part of FIG. 2(b), which corresponds to the high luminance part of the FIG. 2(a), shows a nonlinear shape due to the gamma characteristic .gamma.F of the film. In FIG. 2(b), broken lines represent the output characteristic of the image pickup part 4 obtained in a case where the blackening amount characteristic of the negative film is linear.
As the signal which is as described above is inputted to the processing circuit 5, the white balance of the signal is adjusted. After the white balance adjustment, the signal is subjected to a gamma process with a gamma value of 0.45 for use in the video camera in this instance. Through these processes, the waveform of the signal becomes as shown in FIG. 2(c). As shown, the high level part of FIG. 2(b) is compressed. In FIG. 2(c), broken lines show the levels obtained before the gamma process. The R and B signals are processed in the same manner as the G signal. After this, a matrix process is performed to generate the signal Y and the signals R-Y and B-Y. Since the matrix process is of a so-called linear matrix type, the characteristic obtained before inputting to the matrix is reflected as it is in the output.
In other words, as shown in FIG. 2(d), the signal Y is inverted into a shape which is as a whole compressed from the original level of the object 1. The shapes of the color-difference signals R-Y and B-Y of course likewise become compressed shapes. Then, when these signals Y, R-Y and B-Y are applied respectively to the inversion circuits 6, 7 and 8, they are inverted in their compressed states. Their levels thus become lower than normal output levels as shown in FIG. 2(e). As a result, an image thus obtained on a picture plane is dark as a whole, weak in contrast and faint in color. Besides, since the gamma value of the film does not coincide with that of the video camera, the gradation of the image also becomes unnatural.
As is well known, a document camera apparatus (a video visualizer) has been commercialized as a camera appliance. The document camera apparatus is arranged to pick up, from above, an image of a document or the like placed on a copyboard, to output the image to a TV monitor or to a computer. The document camera apparatus is usable not only for things written on a paper sheet but also in picking up an image of a three-dimensional object for a presentation.
The conventional document camera apparatus which has been used for the above-stated purposes has presented a problem, which is as follows. Since a video camera is usually used for the document camera apparatus, a gamma process is applied to the level of the signal of the picked up image. However, the gamma value for the video camera is normally set at 0.45. As a result, a high level signal is compressed. The gamma correction which is made in this manner generally gives a large dynamic range in picking up an image of an ordinary natural thing. However, in a case where an image of a document or the like is to be picked up by means of an ordinary video camera, the gamma correction tends to result in a compressed contrast to impair the legibility of letters included in the document.